These experiments are designed to find out fundamental characteristics of muscle as a converter of chemical potential energy into mechanical and other forms of work. We have made considerable progress in establishing conditions and muscle preparations in which there is a full and complete accounting of the energy input-output relationships; that is, conditions are known wherein an energy balance is achieved. This work is essential because it is a prerequisite for further study. Secondly, we are making experiments to measure the cost of processes other than actomyosin interaction associated with contractile activity and recovery metabolism. However necessary these energy costs are to the overall function of muscle, they represent a kind of "inefficiency" for work performance. An example is the covalent phosphorylation of myosin and other enzymes as a consequence of contractile activity. Finally, we are studying energetics of working amphibian and mammalian muscles to measure the net efficiency of chemomechanical transduction and to test whether this efficiency depends on experimental conditions and is under control of a variable physiological parameter.